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How to clean masonry

It is important to first assess whether cleaning will benefit the building, as it can cause damage, either while cleaning is underway, or over a period of time after completion.

The type of dirt or other blemish and the material to be cleaned should be assessed and, in some cases, trials carried out. The client should be consulted on this process so that they are aware of the likely results and accept the quality standard of the work.

The most common cleaning agent is water, however, this can create problems (such as decay) if it isn’t used carefully, with the most appropriate chemicals. A typical technique is to apply water to the surface using a pressure washer. Pre-soaking the masonry fills its microscopic pores with water, keeping the chemicals on the surface and guarding against deeper penetration. A white residue can be left behind if the chemicals are not fully flushed out when the building dries.

Fungus, moss and mildew can generally be cleaned by using household liquid bleach in warm water, applied with a stiff-bristle brush. The solution must be rinsed off with clean water. As bleach can struggle to dissolve large amounts of growths, a broad-bladed putty knife may be used to scrape off as much as possible from the surface before applying the bleach. The bleach should be left for a short while before scrubbing with a brush and rinsing to properly remove the fungus spores which can quickly grow back.

A slightly more difficult cleaning problem is presented by oils, soot and mineral residue, which are often embedded deeper into the masonry pores than moss or mildew. A solution of muriatic acid and water can be applied, scrubbing off after around 15 minutes before rinsing.